The present invention relates to microlithography, which is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as an energy beam.
Conventional apparatus for performing charged-particle-beam (CPB) microlithography are represented by various electron-beam microlithography systems such as Gaussian spot-beam systems, variable-shaped beam systems, cell projection-exposure systems, and shaped-beam exposure systems. In many of these systems, the maximum field size that can be exposed is relatively small, on the order of 10 xcexcm square (on the substrate) per exposure. More recent divided-reticle electron-beam microlithography systems have been developed that can expose a pattern divided into square sections (xe2x80x9csubfieldsxe2x80x9d) as large as 250 xcexcm square on the substrate, wherein the various subfields are exposed individually in a sequential manner. Current research also is directed to the development of ion-beam microlithography systems that can expose even larger regions per xe2x80x9cshot.xe2x80x9d
One technical challenge to exposing larger regions per shot is adequate control of the Coulomb effect. The Coulomb effect is manifest as image blur due to Coulomb interactions (repulsion) between individual charged particles in the charged particle beam that causes the particles to scatter. Adequately controlling the Coulomb effect allows higher resolution exposures to be made with minimal blurring, even when using a higher illumination-beam current than in prior systems.
Unfortunately, exposing a larger region typically is accompanied by a more prominent space-charge effect (in which the charge distribution created by the charged particle beam produces its own lens action). The space-charge effect is especially important when using CPB microlithography to fabricate semiconductor devices having a minimum linewidth of 0.1 xcexcm or less. In such applications, a failure to correct aberrations due to the space-charge effect can seriously degrade the performance of the semiconductor devices produced.
Changing the beam current can change image magnification and focus due to the space-charge effect. The degree of image defocus (referred to as xe2x80x9cCoulomb defocusxe2x80x9d) varies with certain parameters such as pattern-element density. Coulomb defocus of a particular image can be corrected by repositioning the focal point of the beam. However, in divided-reticle microlithographic exposure, a large number of subfields are exposed to transfer an entire pattern. Each subfield typically has a different distribution of pattern elements. Consequently, the need to perform focus and magnification alignment for each respective subfield constitutes a major problem. For CPB microlithography systems, the ability to predict and correct, accurately and rapidly, changes in the image due to the space-charge effect is a critical requirement for a practical high-resolution CPB microlithography system.
One system proposed as a solution to this problem is disclosed in U.S. Pat. No. 6,087,669. This system corrects changes in image magnification, rotation, astigmatism, and distortion due to the space-charge effect as caused by changes in beam current and by differences in the distribution of pattern elements from subfield to subfield. However, this device falls short of achieving more accurate exposures due to its inability to make more accurate corrections of aberrations arising from the space-charge effect.
Conventionally, little consideration is given to converting integrated-circuit design data into exposure-correction data useful for correcting aberrations due to the space-charge effect. There also is a marked lack of contemporary knowledge of how to go about computing data for making accurate corrections of the space-charge effect in microlithography systems such as divided-reticle systems in which the distribution of pattern elements differs from one subfield to the next.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide charged-particle-beam (CPB) microlithography apparatus, methods, data-conversion methods, reticles, and semiconductor-device manufacturing methods in which high-resolution exposures are obtained, especially exposures in which aberrations caused by space-charge effects are reduced substantially.
To such ends, and according to a first aspect of the invention, CPB microlithography apparatus are provided for transferring a pattern, defined on a reticle segmented into multiple exposure regions, onto a sensitive substrate. An embodiment of such an apparatus comprises an illumination-optical system, a projection-optical system, a beam-correction-optical system, and a control computer. The illumination-optical system is situated and configured to direct a charged-particle illumination beam from a source to a selected exposure region on the reticle. The projection-optical system is situated and configured to direct a charged-particle patterned beam, formed by passage of a portion of the illumination beam through the exposure region, from the exposure region onto the sensitive substrate, so as to form a transfer image of the exposure region on a selected corresponding region of the substrate. The beam-correction-optical system is situated and configured to correct the transfer image based on correction data for correcting a space-charge-effect (SCE)-based aberration. The beam-correction-optical system also can correct conventional aberrations, e.g., image-field-curvature, and astigmatism, based on conventional methods. The control computer is connected to the beam-correction-optical system and is configured to control the beam-correction-optical system from the following types of input data: (a) the distribution of pattern elements in the exposure region, (b) the illumination-beam current, (c) the spread-angle distribution of the illumination beam, (d) the beam-accelerating voltage to which the illumination beam is subjected, (e) the axial distance between the reticle and the sensitive substrate, and (f) optical characteristics of the projection-optical system. A processor (or multiple respective processors) normally is used to calculate correction data from the input data. The processor(s) can be integrated into the CPB microlithography apparatus or, alternatively, provided separately from the CPB microlithography apparatus, wherein only the calculated correction data (from the processors) are input into the CPB microlithography apparatus. I.e., the processors(s) or software controlling them can be provided separately as separate products from the CPB microlithography apparatus itself.
The exposure regions on the reticle can be in the form of subfields each defining a respective portion of the pattern, wherein each exposure region is substantially coextensive with a respective subfield. In such a configuration, the control computer is configured to cause the illumination beam to illuminate the subfields sequentially and to cause the patterned beam to transfer images of the respective pattern portions defined within the subfields to the sensitive substrate in sequence.
Alternatively, the exposure regions on the reticle can be in the form of deflection fields each defining a respective portion of the pattern. In such a configuration, the control computer is configured to cause the illumination beam to scan the deflection fields sequentially and to cause the patterned beam to transfer images of the respective pattern portions defined within the deflection fields to the sensitive substrate in sequence.
The beam-correction-optical system desirably is configured to correct at least one of rotation, magnification, focal point, astigmatism, anisotropic magnification, orthogonality, and position of the transfer image. Further desirably, the beam-correction-optical system corrects more than one of these characteristics. To such ends, the beam-correction-optical system desirably comprises at least three focus-correction lenses, at least two stigmators, and at least one deflector. Such a configuration allows independent correction of parameters selected from image rotation, image magnification, image focal point, image astigmatism, image anisotropic magnification, image orthogonality, and image position. The focus-correction lenses desirably are used to correcting focus, rotation, and magnification of the image. Each of the stigmators desirably has a multi-pole configuration. The stigmators are used for correcting astigmatism, orthogonality, and anisotropic magnification of the image. The deflectors desirably are used for adjusting the position of the image on the sensitive substrate. The use of a combination of three focus-correction lenses enables rotation, magnification, and focus of the image to be adjusted simultaneously as desired. The combination of at least two stigmators enables variations in astigmatism, orthogonality, and anisotropic magnification to be corrected simultaneously as desired. Hence, multiple corrections can be made at the same time, even when the noted aberrations occur in arbitrary ratios (relative to each other) depending upon the SCE-related parameters such as distribution of pattern-element openings within the transfer subfield, illumination-beam current, illumination-beam spread-angle distribution, illumination-beam acceleration voltage, axial distance between the reticle and substrate, and optical characteristics of the projection-optical system.
With respect to the focus-correction lenses, all can be electromagnetic, two can be electromagnetic and one electrostatic, or one can be electromagnetic and two electrostatic. With respect to the stigmators, both can be electromagnetic or electrostatic, or one can be electrostatic and the other electromagnetic. These combinations can be changed as appropriate for use with electromagnetic deflectors or electrostatic deflectors.
The apparatus desirably further comprises a memory situated and configured to store the correction data. The control computer recalls the correction data as required for routing to a drive system connected to the beam-correction-optical system. Thus, the beam-correction-optical system is driven (actuated) according to the recalled correction data. Including a memory for storing correction data allows rapid correction of SCE-based aberrations.
According to another aspect of the invention, methods are provided for performing CPB microlithography in which an exposure region on a divided reticle is illuminated by an illumination beam, and a patterned beam propagating downstream of the illuminated region is imaged by a projection-optical system onto a surface of a sensitive substrate. The subject methods include a method for correcting a space-charge-effect (SCE)-based aberration arising while transferring an image of the illuminated region onto the substrate surface. In the correcting method, a calibration pattern is used. An expected SCE-based aberration is computed that is expected to occur if the image of the calibration pattern were transferred to the sensitive substrate. The calibration pattern is illuminated with the illumination beam so as to form an image of the calibration pattern on the sensitive substrate. An actual SCE-based aberration exhibited by the transferred image of the calibration pattern is measured. From the measured SCE-based aberration, a correction coefficient is computed for correcting a difference between the expected SCE-based aberration and the actual SCE-based aberration of the transferred image of the calibration pattern. Correction data are determined for correcting an SCE-based aberration of a transferred image of an exposure region of the reticle. The correction data are calibrated according to the correction coefficient. Based on the calibrated correction data, a transfer image is corrected and projected onto the sensitive substrate.
In the foregoing method, the step of measuring an actual SCE-based aberration desirably comprises at least one of: (a) producing an electrical signal from projecting the calibration pattern onto a detection mark provided on the sensitive substrate or on a substrate stage, and analyzing the signal with respect to one or more representative points of the calibration pattern; and (b) developing the image of the calibration pattern on the substrate, and measuring the image using a measuring instrument.
Further with respect to the foregoing method, the step of computing an expected SCE-based aberration can be performed based on the following parameters: the distribution of pattern elements within the calibration pattern, the beam current of the illumination beam, the spread-angle distribution of the illumination beam, the accelerating voltage of the illumination beam, the axial distance from the reticle to the substrate, and the optical characteristics of the projection-optical system. The computed expected SCE-based aberration and the measured SCE-based aberration desirably comprises at least one of: image rotation, image magnification, image focal point, image astigmatism, image anisotropic magnification, image orthogonality, and positional displacement of the image.
According to another aspect of the invention, data-conversion methods are provided. The data-conversion methods are especially applicable to methods for performing CPB microlithography in which an exposure region on a divided reticle is illuminated by an illumination beam passing through an illumination-optical system. A patterned beam propagating downstream of the illuminated exposure region through a projection-optical system is imaged on a surface of a sensitive substrate. The data-conversion method is applied to converting data used for computing an amount of correction of an SCE-based aberration in an image of the exposure region on the sensitive substrate. In an embodiment of the data-conversion method, the SCE-based aberration is computed from data concerning: the distribution of pattern elements within the exposure region, the illumination-beam current with which the exposure region is illuminated, the spread-angle distribution of the illumination beam, the accelerating voltage of the illumination beam, the axial distance from the reticle to the sensitive substrate, and optical characteristics of the projection-optical system. Aberration-correction data are produced from the computed SCE-based aberration.
The step of computing the SCE-based aberration can be performed by: (a) computing a distribution of space-charge created by the patterned beam propagating between the reticle and the sensitive substrate, (b) computing the electrostatic potential created by the distribution of space-charge, and (c) re-computing the trajectory of at least one of the illumination and patterned beams, or computing the displacement of the trajectory of at least one of the patterned beams, based on the computed electrostatic potential. The step of computing the distribution of space-charge can comprise the steps of: (1) expressing a spread-angle distribution of the illumination beam as a function A(x,y,z), wherein z is a parameter corresponding to a position on the optical axis of the illumination- and projection-optical systems, and x and y are respective parameters corresponding to respective positions in an xy plane perpendicular to the optical axis; (2) defining a convolution integral of the spread-angle distribution A(x,y,z) and a distribution Pa(x,y) of pattern elements within the exposure region; and (3) computing discrete values of the space-charge according to the convolution integral, wherein corresponding values of the spread-angle distribution A(x,y,z) are recalled from a look-up table previously stored in a memory, wherein the table contains definite integrals of the x and y parameters of the spread-angle distribution A(x,y,z). The step of computing the distribution of space-charge can comprise computing a near-axis trajectory of the patterned beam passing through the projection-optical system.
A representative calculation for determining the density distribution of space-charge xcfx81(x,y,z) of an arbitrary point (x,y,z) in the charged particle beam is as follows:
xcfx81(x, y, z)=k*(1/rb(z))2
*(∫∫A(X, Y, z)dXdY)xe2x88x921*∫∫Pa
((x1*cos xcex8b(z)+y1
*sin xcex8b(z))/rb(z), (y1
*cos xcex8b(z)xe2x88x92x1*sin xcex8b
(z))/rb(z))*A
(xxe2x88x92x1, yxe2x88x92y1, z)dx1dy1
wherein ra(z), rb(z), xcex8a(z), and xcex8b(z) are the radial and rotational components of representative trajectories Wa and Wb, and k is the illumination-beam charge density immediately upstream of the reticle. Hence, the charge density xcfx81(x,y,z) is the convolution integral of the spread-angle distribution A(x,y,z) of the illumination beam and the pattern-element distribution Pa(x,y).
Further with respect to the data-conversion method, the distribution of space-charge can be computed by: (a) dividing the exposure region into sub-subfields; (b) quantifying one or more characteristics of the pattern elements existing within a sub-subfield; (c) configuring simplified graphic figures having characteristics corresponding to the quantified characteristics of the pattern elements; and (d) computing the distribution of space-charge based on the simplified graphic figures. The characteristics of the pattern elements include at least one of: (i) the area of all pattern elements within a sub-subfield, and (ii) the centroid of all pattern elements within the sub-subfield. The area and centroid of pattern elements existing within the sub-subfield desirably are calculated as follows:
Area=xcexa3Sj
Centroidx=(xcexa3GxjSj)/(xcexa3Sj)
Centroidy=(xcexa3GyjSj)/(xcexa3Sj)
wherein the summations (xcexa3) are made over all pattern elements existing within the sub-subfield, Sj is an area of a pattern element j existing within the sub-subfield, j is an integer equal to the number of pattern elements, and Gxj and Gyj are x and y coordinates, respectively, of a location of the centroid of the pattern element (j) in a plane of the pattern.
The step of computing the SCE-based aberration can be performed as follows. A first-order approximation of the SCE-based aberration in the image of the exposure region is computed by: (1) computing the electrostatic potential created by the distribution of space-charge, based on a computation of the near-axis trajectory of the patterned beam passing through the projection-optical system; and (2) recomputing the trajectory of the patterned beam based on the computed electrostatic potential. A second-order approximation of the SCE-based aberration in the image of the exposure region is computed by: (1) using the re-computed trajectory of the beam, recomputing the distribution of space-charge created by the patterned beam propagating between the reticle and substrate; (2) computing the electrostatic potential created by the recomputed distribution of space-charge; and (3) recomputing the trajectory of the patterned beam based on the computed electrostatic potential. These steps are repeated as required until a difference between the computed first- and second-order approximations is within a prescribed tolerance.
In executing the data-conversion method summarized above, calculation of the SCE-based aberration can be performed using a computer system comprising multiple processors. Each processor performs the computation of an SCE-based aberration for a different respective exposure region. The computations performed by the processors are performed in parallel.
The aberration-correction data desirably comprise a numerical value for at least one of the following: image rotation, image magnification, image focal point, image astigmatism, image anisotropic magnification, image orthogonality, and positional displacement of the image. Furthermore, the aberration-correction data desirably include one or more of the following: (i) data used for controlling actuation of a CPB-correction-optical system so as to correct the SCE-based aberration, and (ii) data used for altering a position and/or shape of a pattern element defined by the reticle in a manner serving to cancel the SCE-based aberration when the pattern element is imaged onto the substrate. The data used for altering a position and/or shape of a pattern element defined by the reticle can comprise data for canceling at least one SCE-based aberration selected from the following: image rotation, image magnification, image orthogonality, image anisotropic magnification, image position displacement, and higher-order distortion of the image.
According to another aspect of the invention, CPB microlithography apparatus are provided that include a projection-optical system situated and configured to direct a patterned beam from a reticle to a substrate. An embodiment of the apparatus also includes a CPB-correction-optical system and a control computer connected to the CPB-correction-optical system. The control computer is configured to drive the CPB-correction-optical system based on data, to be used by the CPB-correction-optical system, produced by a data-conversion method according to the invention.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.